Expositions of the prior art are to be found in U.S. Pat. No. 4,008,601 (Woods) and in published U.K. patent application No. 2016278A of the present applicants.
Briefly stated, the Woods U.S. Pat. No. 4,008,601 discloses a flueric partial pressure sensor that enables the partial pressure of a constituent gas of a gaseous mixture to be measured directly relative to a reference-gas by ensuring that the pressure drop output of a flueric bridge concentration sensor is made constant whereby the output pressure signal of the sensor is directly proportional to the partial pressure of the constituent gas at one given absolute pressure.
A sensor according to U.S. Pat. No. 4,008,601 utilises a flueric bridge gas concentration sensor such as is disclosed in U.S. Pat. No. 3,771,348 (Villarroel) having first and second flow channels of equal flueric resistance for receiving a reference-gas and a sample-gas, respectively. Reference and sample-gas flows through the respective first and second flow channels are generated by an aspirator which is essentially operated in its sonic region, or choked flow condition, in order that the reduced pressure created in its aspirated inlet is directly proportional to the ambient pressure. By this means the output pressure of the sensor is directly proportional to the partial pressure of oxygen, or other constituent in the sampled gas mixture.
A disadvantage of a partial pressure sensor arranged in such manner for use in a variable ambient pressure condition is that its pressure signal output for a given partial pressure varies with change in the ambient pressure, i.e. its pressure signal output varies with change in altitude for a given partial pressure of oxygen.
Also briefly stated, the disclosure of U.K. patent application No. 2016278A is directed to apparatus providing breathable gas delivery regulators for working on low supply pressures, below 10 psi (69 kPa), comprising a gas inlet for receiving a breathable gas and a gas outlet for connection to a user between which a demand valve is situated for controlling the flow of gas to the user. A pressure sensor is included which is responsive to the user's breathing pressure and is connected to a control port of a fluidic amplifier of a servo arrangement. The output of the amplifier is connected to an actuator for the demand valve so that the valve is operated in response to breathing pressure signals from the pressure sensor. In a favoured form the regulator includes a breathable gas proportioning valve which allows either of two gases or a mixture thereof to be supplied to the gas inlet and furter includes a flueric gas mixture composition detector means for providing a pressure signal significant of the content of one of the gases in the mixture. This pressure signal is applied as a regulating feedback signal to an actuator of the gas proportioning valve which operates in response to the difference between ambient pressure and absolute pressure.
A component is included in such a regulator for providing absolute pressure to the actuator of the gas proportioning valve and is preferably in the form of an absolute pressure sensor comprising a high recovery venturi with means for inducing a choked flow of ambient air therethrough via a passage of constant cross-section having a tapping for detecting the pressure in the passage to obtain an absolute pressure.
A nuisance value is attributable to the absolute pressure sensor in terms of weight, space and effort required to operate it. All these terms are obviated by use of the present invention which removes the need for such a sensor.
According to one aspect of the present invention a flueric partial pressure sensor including a flueric bridge having two bridge legs adapted for sensing a reference-gas and a sample-gas mixture, respectively, is characterised by being arranged to provide a constant pressure output signal for a predetermined constant partial pressure value of one of the gases in the sample-gas mixture at varying absolute pressure.
In an embodiment of the invention a linear resistor and an orifice resistor are incorporated in each of the two said bridge legs which are conjoined to discharge from a single outlet. Respective pressure signal outlets are connected one with each leg at a position between the linear resistor and the orifice resistor, and the resistors are arranged to provide an asymmetric balance of the flow rates through the two said legs. The asymmetric balance is selected so that, in operation, a constant, or substantially constant, pressure output signal is generated for a chosen partial pressure of a constituent gas of the sample-gas mixture in varying or variable absolute pressure conditions such as changes in altitude.
Extensive experimentation in our laboratories has shown that for a flueric bridge partial pressure sensor having a symmetrical or balanced bridge with air supplied as a reference-gas to one leg and oxygen-enriched air supplied as a sample-gas to the other leg, the pressure output signal changes in direct proportion to the percentage oxygen content of the enriched air providing that the sensor is operated at a fixed absolute pressure or altitude. On the other hand, we have found that with such an asymmetrical bridge circuit bleeding to the control regions of a suitably matched high input impedance flueric laminar flow proportional amplifier the differential output pressure from the amplifier will remain at a constant value over a wide range of absolute pressure conditions (e.g. altitude) when the partial pressure of the sample gas remains constant.
The present invention, in preferred form, introduces a small degree of asymmetry in the flow rates through the reference-gas and sample-gas legs of the bridge sensor and also asymmetry in the relative length/diameter ratios of the orifice resistors which are arranged so that the changing values of their relative coefficients of discharge at various operating pressure ratios causes a degree of differential pressure output that will vary with changes in absolute pressure when identical gas mixtures are passed through the two legs of the bridge.
Accordingly, another aspect of the present invention provides a flueric partial pressure sensor including a flueric bridge having one leg adapted for receiving a reference-gas and another leg adapted for receiving a sample-gas mixture, each said leg including a linear flow resistor and an orifice flow resistor serially arranged therein and being operably connected to flow inducement means for drawing said reference-gas and said sample-gas mixture therethrough, characterised in that said flow resistors are arranged to cause asymmetric flow rates through the respective legs, the orifice resistors providing a small degree of asymmetry in their relative coefficients of discharge so that changing values of coefficients of discharge at various operating pressure ratios causes a degree of differential pressure output from the bridge which varies with changes in absolute pressure when identical gases or gas mixtures are drawn through the bridge legs.
The out of balance, i.e. degree of differential pressure output, can be eliminated when the subject constituent of a gas mixture in the sample leg relative to the other gases therein is re-proportioned so that, despite the overall change in the total pressure of the mixture, the partial pressure of the subject constituent remains constant.
The linear resistors in the legs of the bridge sensor may be positioned either upstream or downstream of the orifice resistors; however, in respect to sensing the partial pressure of oxygen in oxygen-enriched air, it is preferable that the linear resistor is the upstream element of the two, because the reverse arrangement is unnecessarily sensitive and tends to give an unstable device.
The pressure output signal is, in practical usage, amplified and fed as a driving or control signal to pressure responsive means, such as a gas mixing valve or a partial pressure variation indicator device.
As the pressure output signal is in two parts, being derived from the reference-gas leg and the sample-gas leg, it is convenient to use this to operably control the main jet of a flueric amplifier.